Old school method is to kräusen it. You would add a portion of actively fermenting beer to your priming bucket. You can also do this with plain wort, but the fresh and healthy yeast in actively fermenting beer has a few benefits... clean up diacetyl, acetylaldehyde, and other fermentation by-products.
Here's a formula for how much gyle to add: priming with gyle formula
Here's a calculator for how much gyle to add: priming with gyle calculator
